Self-focusing, direct-coupled, even stage amplifier



Jan. 16, 1951 w. K. VOLKERS 3 SELF-FOCUSING, DIRECT-COUPLED, EVEN STAGE AMPLIFIER Filed Dec. 31, 1946 2 Sheets-Sheet l Prior fir-7 INVENTOR.

WaH'c-rr Vo/kers Jan. 16; 1951 w. K. VOLKERS 2,538,487

SELF-FOCUSING, DIRECT-COUPLED, EVEN STAGE AMPLIFIER Filed Dec. 31, 1946 2 Sheets-Sheet 2 vINPUT oL/TPu-r INVENTOR.

Wahei K. Volkers BY A-QJW #orney Patented Jan. 16, 1951 t Walter K. Volkers, Schenectady, n. in, assi no to iVolkers & Schafier, Inc .a corporation of New York ApplicationiDecember 31, 194$,Seniail No. 71 93305 5 '(ila'ims. (cum-i111 The present invention concerns an :exceptionrally-simple self-focusing circuit ior direct coupled even-stage amplifiers, particularly two-stage amplifiers. Until its discovery direct coupled amplifiers for audio frequencies and other purposes were invariably more complicated and expensive i heir=design than the standard ,R/C type am- ',p1-ifier, and their use was therefore restricted to laboratory work and similar special purposesn -Ihc main object of the present invention is to snake available to the radio industry a self focusing directecoupled amplifier which requires less parts and is more economic in production thanR/C coupledamplifiers, thus helping to reduce (the manufacturing costs .of domestic radios and similar equipment;

"Another object of my invention is to provide an improved self-focusing circuit for direct-coupled amplifiers which possesses better hum and voltage-fluctuation -compensa't'ion properties than fearlier designs in this field.

A further object of my invention is to avail myself of the peculiar low-screen-voltage/1ow- ';scre,en-.current properties of starved screen grid 'Itu-bes for the purpose of eliminating screens "filtering equipment.

This starved operation is fully described in my co-pending patent applica- "tion Serial No. 701,989, filed October 8,, I946,

"fHigh-ga'in low-dra'in operation lot screen-grid tubes? Finally, an object of my 'ihvent'ionis to provide :an unusually simple direct-coupled two-stage ramp-lifter which has a noticeably higher overallgain'than conventional *R/C coupledamplifiers.

invention will be better understood by re- "ferring to the accompanying drawing in which: "Figure 1 shows a standard R/C coupled amplifier having two condensers and two resistors in its output stage,

Figure "-2 the nearest prior art 'to'my invention disc'losing a self-focusing direct :coupled amplifier which also "requires two condensers and "two reasistors in its output stage,

' Figure 3 1a self-focusing direct coupled .ampiiizfier according to my invention requiring one "tapped resistor and one by-tpass condenser only in the output stage, and

.Etgure 4 another form :of ,my invention which has an untapped resistor and one condenser in the output stage but a tapped cathode resistor in the input stage.

Figure 5 shows a combination of these designs and r V Eigs. 6 7 the application of loose-grid :stanvation" as additional means of focusing 1;-

cent-coupled amplifiers. I

The circuits :inuFieures ":1 and 2 cleanly reveal reasons why even the simplest selieioousins direct coupled amplifieriknown in the art and merroducedinflis. ;2 for comparisontpmp sesba been unable ire-compete commercially with Ev/ C coupled-amplifiers. 4 I I The standard non-degenerative ,R/fiqfcoupied amplifier iinvsrlinune- Ll-has its -ou put stage a "G t flQdedQSiStOI' i, bit-79 155? by ascondens rez.

also theusualsrid-leak 13, asezvella phid-scoutiling -;condenser =4, the competin direct coupled amplifier :in :Fi ur 2 has inots, output-stage cathode re istor 1 VI which is divided iby tap A2 i t an upperrsiection 11-3 and a lower section M.

1-2 supplies the .fselfdoeusin potential-It "thescreen-it of the input tuberlfi e. a filtered I ne atiue-rf-eedbacklg. \C, potential. fllhe poten tial of this screen is stabilized a ainst voitage fluctuations by the filter condenser lJ- which :is connected from the screen l;5 toiground. :Suoh stabilization-is needed :for two reasons:

to) Since the (cathode :21! of the output tuned! is ,not Icy-passed to ground gbut-capacitiyel-y 011- pass-condenser of equivalent suitable .size.

pled to the positive supply terminal to, the rout- :tube is ibDllIl-d"zt0 operate by itself semi-idegeneratively .for reasons which will-he-texplained helow. This means thatza portion-oi the audio signal will (appeariat tap 12 of the cathode re- ,sistor H from where it would be; fed back degeneratively through both tubes yia ascreen J5 unless condenser 11 stabilizes the potential of the screen.

The prior-iartecimuit in .Eigure -12 avails itself of this means ref-stabilization and, in addition, stabilizes to some extentthe potential of cathode (29 ofthe output tube .26! .ithuough the hyip-ass condenser 19 ihetween the cathode vand I the positive supply terminal I8.

stabilization can :b :only ;partia 1, -since, in

1 order to obtain a realiyefiectiyezor tull stabilizes tion of the cathode potential Jo y-pass condenser .19 and in addition the-output power supnlysiiltercondenser 22 (Which.:a1 e-.bo'.thconnected miseries I asfar as the Irv-passing of cathode :20 to ground is concerned) would have to he soilar ge that their series-capacity replaces (a .ca'thodeto ground thy- The latter would he required to be substantially .iarger than conventional cathode bypass-condensers, since the cathode resistor l i is bound tobelarger than ordinary output tube cathode {resistors Able- 1cause-as a result oi -the direct-couplingabetween the tubes, it has-to :match -.both {the output vtubefs bias and, in addition, the input tubeis plate Moltage. ,A.50L6-output-tube,i$or instance, would-non mall-y require a biasing cathode resistor .ofiapproximately ohms. When direct-coupled .to

this condenser alone does not by-pass the cath- 1 ode to ground. It will be necessary, instead, to

-- make in addition, condenser I9 100 microfarads,

preferably 200 microfarads, and condenser 22 of the plate supply also 100 microfarads, preferably 200 microfarads, in order to obtain the necessary cathode to ground by-passing capacity of 50-100 mf. This means a very substantial increase of manufacturing costs since condenser 22 would have to be enlarged substantially beyond the customary 8 microfarads (with filter-choke) or 30 7 to 40 microfarads (with filter-resistor). Furthermore,-this large condenser 22 as well as condenser 19 would have to be insulated for the full plate supply voltage which also increases their costs way beyond the costs of the simple low-voltage 1 cathode by-pass condenser 2 of the R/C amplifier in Fig. 1.

(b) The other reason why screen-grid l5 must .-be stabilized against voltage fluctuations by means of condenser I1 is that cathode by-pass I J condenser I9 in Figure 2 transmits voltage fluctuations of the plate-supply terminal I8 to the cathode. With the cathode resistor H acting as a voltage divider these fluctuation would reach the screen and would be amplified by both tubes. If the plate supply is a rectified A. C. source this would mean a transmission and amplification of its hum through the amplifier. Any

other kinds of voltage fluctuations would exert a similar disturbing influence.

The circuit in Figure 3 is, according to my invention, free from these drawbacks. The output tubes cathode resistor 3| is by-passed by a low voltage medium-size condenser 32, similar in rating to condenser 2 of the R/C amplifier in Figure There is no transmission of plate-supply hum to screen present through a condenser. The only manner in which the plate-supply hum can reach the screen of the input tube is through the outputtubes plate-current and the corresponding voltage drop across cathode-resistor 3|. l Tl hum-compensating theory in my co-pending patent application Serial No. 701,997 filed October .8, 1946, however, clearly discloses that whatever .Yi 'hum appears in cathode 33 of output tube 34 in Figure 3 will, through its re-entry into the circuit screen 35 of input tube 36, have a degenerating for self-compensating effect which will improve I the amplifiers performance.

The

Exactly the opposite is, however, the case as far as hum re-amplification in the earlier selffocusing circuit in Figure 2 is concerned. Here the low-impedance-connection between the platesupply l8 and cathode l3, represented by the.

- large condenser l9, causes heavy hum-over-compensation and the consequent hum-increase through amplification via screen l5.

Obviously, my new self-focusing direct-coupled voltages is concerned but, in addition, it also offers an improved performance with regard to hum-compensation and compensation against line voltage fluctuations.

It shall be clearly understood that a condenser similar to the screen condenser H in Figure 2 does not appear in my new circuit in Figure 3 for several reasons which are beyond a simple omission of a part at the expense of a minor or major loss of performance or quality. Instead,

the screen condenser ll of Figure 2 is deliberately omitted in Figure 3 as a result of the following physical facts the recognition of which I consider major points of my invention:

1 2 This condenser acts simultaneously as a stabilizer for the potential of cathode 33 of output tube 34 and the screen 35 of the input tube 36.

electrodes.

(11) Since cathode 33 of output tube 34 in Figure 3 is more effectively ground-by-passed than cathode 20 of tube 2| in Figure 2 (the reasons for this improved by-passing having been fully stated above) negative audio-feedback from the output cathode resistor 3! in Figure 3 to the input screen 35 is negligible or can be made so without involving unreasonable manufacturing costs. Additional screen-filtering equipment can therefore .be omitted.

(b) The positive plate-supply 31 in Figure 3 does not inject directly through a condenser a large ripple or hum component into the outputcathode to input-screen feedback circuit so that no precautions against amplified feedback hum need be taken.

(0) The cathode resistor 3| of the output tube is a comparatively husky, low-ohm device which acts as a highly stable voltage divider with regard to screen 35 of'the input tube keeping that screens potential at certain values regardless of potential variations of the input tubes control grid 38 or changes of the input tubes plate current.

This is particularly, true if the'output tube is a power tube requiring cathode resistors of only a few hundred ohms or less. 7 v

(d) By operating the input tube 36 under a starved condition, 1. e. with a plate-current of less than of the all-cold-electrodes-tocathode-discharge-current screen currents are, according to my co-pending case IPA-9913 Serial No. 701,989, reduced to an almost undetectable fraction of their value in the unsta'rved condition. It will'be understood that the expression "all-cold-electrodes to-cathode-dischargecurren is the-current which would flow from the cathode of the tube to the cold electrodes where all of the cold electrodes are connected together and the tube is connected in a circuit with the normal plate operating voltage applied to the cold Accordingly, the cathode resistor 3| of the output tube assumes an even more husky relative magnitude with regard to a stabilization of the input tubes screen potential and potential variations at tap 39 of resistor 3i follow practically exclusively cathode potential variations of the output tube, without being noticeably infiuenced by screen-current variations of the input tube. This condition, more than all others mentioned, makes it possible to operate the screen 35 of the input tube without screen-filtering equipment. 5

The cathode 46 of the input tube in Figure 3 may be grounded or connected to a biasing resistor or any other circuit element.

As explained in my co-pending applications 11, 18, 22 and 25 Serial Nos.'69'7,584=, and the abandoned applications, 697,585, 101,991, 697,589 and 701,995, referring to direct-coupled amplifiers operating with elevated output cathode potentials, such amplifiers require means of focusing, i. eof adjusting and oorrelatlngthe various tubes and circuit elements to each other insuch manner that the tic-signal output our-rent'as- 's'uin'es a suitable value, usually near the output -tubes focus or approximately half its saturaiti'o'n current.

' *The circuit in Figure '3 may, for instance, be focused by suitalzle selections or adjustments-of the input --tubes load-resistor 5 the output 'tubes cathode-resistor-tap 3-9, the -potentia l the input tubes cathode 40 or its control g-rid etc. A particularly simple method of focusing this amplifier-consists in shifting tap 39 of the output tubeis cathode resistor until the nosignal output current assumes a suitable value hIfhis depri-ves, however, the designer of )his free- *tional means-for "focusing it are provided. Figures 4-7 show such means.

In Figure 4 the screen 35, of the input tube is connected to the cathode 33 of the output tube, leaving no possibility of focusing the amplifier from its output end. Focusing is achieved instead by adequate pre-loading of cathode resistor 4| of input tube 36 through a pre-loading resistor 42 between the cathode 43 of the input 7 tube and its positive supply terminal 44. This method of pro-loading is described in my copending application Serial No. 697,584 filed Sep; tember 17, 1946. It might be replaced by a regular voltage divider or any other means of operating the input tube at the proper negative focusing bias.

Another modification of these circuits is shown in Figure 5, which is a combination of the focusing circuitsof Figs. 3 and 4. the tap on output cathode resistor 3| and the pre-loading circuits of the input cathode resistor ll are variable and the amplifier can be focused through either or both of these circuit elements and, in addition, through the input control grid as and the input load-resistor 5 i, if desired. Such a circuit provides full liberty for operation of the tube at optimum starvation input screen potential without sacrificing focusability or simplicity of design.

Another type of circuit, shown in Figs. 6 and 7, involves zero bias operation through loose grids as means of focusing such amplifiers.

I discovered that pentodes, although, when operated at full, rated screen voltages, draw very small grid-currents only, such grid-currents are considerably increased when the tube is starved. A 6J7 at 15c volts plate-supply and with an 80 volts screen will, for instance, show almost identical grid-plate-current-characteristics with its grid alternatingly connected directly to the controlling potential-source or loosely through a lo, 20 or 50 megohm-resistor. The same tube, with a 10 volts screen, will begin to deviate noticeably from its original characteristics if more than 10,000 ohms are inserted in the gridcircuit. With one or two megohms in the grid lead this tube will behave much like a zero-biasoperated SSQ? triode with a leak of a few megohms. By properly selecting the magnitude of the grid leaks of starved pentodes an additional liberty 6 of design is being provided which can be used for focusing purposes and free selection of star vation screen focusing potentials.

"Figures-6 and 7 show examples :of this designing technique.

in Figure 6 the screen 55 :of the input tube is againconnecteddirectly to the cathode 33 of the output tube and focusing is obtained by properly "selecting or, it preferred, adjusting grid-leak resistor 54=of the input tube in such manner-that the control grid of that tube develops'a suitable negative bias potential through "its grid currents.

Figure 7 is arepetition :of Figure- 6 'excep't that in order to obtain a lower screen voltage the "input screen 35 is connected to a tap 39 on the output tubes cathode resistor.

It w ill'be obvious to those skilled in the ar-t that my invention is not confined to such ch cu'its as "have been discussedand illustrated. 'Numerou-s variations of the principles outlined are possible. Amongst them, for instance, in stead of varying the position of the tap Su te which the i np-ut screen is attached, it is also possible to change in all diagrams of the drawingthe positions of the upper "and lower terminals 52 and 53 of the cathode-by-pass-condenser 32 so that this condenser by-passes only part of the cathode-resistor.

In summarizing my invention, it consists in employing a single negative feedback circuit between the cathode of an output tube and the screen'of an input tube of a two-stage amplifier, no additional means of self-focusing being provided. This single feedback circuit should be equipped with a filtering device allowing D. G.

' potentials to pass, but rejecting the A. C. com- In this case both ponent of the signal. I prefer using as a filtering device the same cathode by-pass condenser which would be required anyhow for the purpose of preventing degenerative operation of the output tube.

In its preferred form although not necessarily in its only form I operate the input tube in a starved condition. Since such condition usually requires screen potentials in the neighborhood of 5-25 volts and since in a direct coupled circuit the voltage drop across the cathode resistors of power-output-tubes is usually in the neighborhood of 10-50 volts, the cathode resistor of such tubes, whether tapped or untapped, provides an excellent potential source for the screen of a starved input tube.v By availing myself of this opportunity a two-stage amplifier could be produced which outperformed the ordinary R/(L coupled amplifier with regard to gain, tone quality and production costs as several samples and tests confirmed.

I claim:

1. A self-focusing, direct-coupled vacuum tube amplifier comprising an even number of amplification stages in which the tube of the input stage has a control grid and a screen grid and the tube of the output stage has a cathode and a power supply having negative and positive ter- ,minals and a resistor connected between its cathode and its negative power-supply terminal, and a single voltage feedback lead for supplying the entire self-focusing energy from the output stage to the input stage comprising a connection least a portion of said resistor and a condenser connected directly in shunt between the cathode of the output tube and the negative power supply terminal, said cathode being connected to the positive terminal of the power supply solely through the space current path of said output tube whereby the transfer of alternating current from said resistor to said screen gridis prevented. 2. A direct coupled self-focusing amplifier according to claim 1, said lead being connected to the cathode end of said resistor.

3. A direct coupled amplifier according to, claim 1, said lead being connected to a tap on said resistor between said cathode of said output tube and said negative power supply terminal.

4. A direct coupled amplifier according to claim 1, said input tube being operated at a plate current less than one-one -thousandth of its all-coldelectrodes-to-cathode-discharge-current measured at a plate-voltage equal to the plate-supply- .voltage at which the tube is being operated.

r 5. A direct coupled amplifier according to claim 1, the cathode of said input tube being connected to'a voltage divider between the positive terminal and the negative terminal of a power supply, establishing a suitable biasing potential of such cathode with reference to said input tubes grids, said biasing potential focusing the output current of said input tube towards its desired oper ating value.-

6. A direct coupled amplifier according to claim 1, said input tube being operated at a plate current less than one-one-thousandth of its all-cold-electrodes-to-cathode-discharge-current measured at a plate voltage equal to the supply voltage at which the tube is being operated, the control grid of said input tube being grounded through a leak resistor larger than vone hundred thousand ohms which produces a negative bias larger than 0.1 volt.

WALTER K. VOLKERS. 

